The subject matter disclosed herein relates to X-ray tubes, and in particular, to X-ray cathode systems and X-ray cathodes.
Presently available medical x-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an x-ray tube anode, or target, which is typically a planar metal or composite structure. The space between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth. In thermionic cathode systems, a filament is included that may be induced to release electrons through the thermionic effect, i.e. in response to being heated. However, the distance between the cathode and the anode must be kept short so as to allow for proper electron bombardment. Further, thermionic X-ray cathodes typically emit electrons throughout the entirety of the surface of the filament. Accordingly, it is very difficult to focus all electrons into a small focal spot.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode. The area where the electron beam impacts the anode is often referred to as the focal spot. Typically, the cathode includes one or more cylindrically wound filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
Conventional cylindrically wound filaments, however, emit electrons in a complex pattern that is highly dependent on the circumferential position from which they emit toward the anode. Due to the complex electron emission pattern from a cylindrical filament, focal spots resulting therefrom can have non-uniform profiles that are highly sensitive to the placement of the filament within the cup. As such, cylindrically wound filament-based cathodes are manufactured having their filament positioned with very tight tolerances in order to meet the exacting focal spot requirements in an x-ray tube.
In order to generate a more uniform profile of electrons toward the anode to obtain a more uniform focal spot, cathodes having an approximately flat emitter surface have been developed. Typically a flat emitter may take the form of a D-shaped filament that is a wound filament having the flat of the “D” facing toward the anode, such as disclosed in U.S. Pat. No. 7,795,792 B2, incorporated herein by reference in its entirety. Such a design emits a more uniform pattern of electrons and emits far fewer electrons from the rounded surface of the filament that is facing away from the anode (that is, facing toward the cup). D-shaped filaments, however, are expensive to produce (they are typically formed about a D-shaped mandrel) and typically require, as well, very tight manufacturing tolerances and separately biased focus electrodes in order to meet focal spot requirements.
Thus, in another example of a flat surface for forming a filament, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament.
In addition, recent developments in diagnostic x-ray tubes made it desirable to provide high emission at reduced tube voltages. For example in vascular x-ray tubes it is desirable to reduce tube voltages to 60 kV from the typical lower limit of 80 kV while ideally maintaining the power delivered to the target. For large focal spots, emission currents between 1000 mA and 1500 mA at 60 kV are desirable. For small focal spots, especially in fluoroscopic mode, emission currents up to 400 mA are desirable. In both cases, the current state of the art, for flat emitters only allows about half the desired emission current.
As larger rectangular emission areas are required to enable higher emission it becomes more challenging to focus the electron beam into a small spot. Further, as shown in FIG. 1, with the dots indicating electron trajectories intersecting the target plane and the stitched lines indicating iso-density contours of the focal spot, the focal spot resulting from the rectangular flat emitter has a shape with some significant aberrations from an ideally straight intensity distribution. The distortion results in the four corners of the focal spot stretched away from the focal spot center thus increasing the effective size of the focal spots, especially in width direction.
To address these focal, spot issues, electrodes are often utilized to direct the electrons from the emitter towards are more defined focal spot. As shown in FIGS. 2 and 3, small and large emitters are surrounded by width and length electrodes that are typically positioned on the cathode cup a few millimeters above the plane of the emitter. The focusing fields provided by the electrodes can direct electrons from the emitter into a more defined focal spot when the electrons are emitted, within a region that the electron can be effected by the focus pads. In FIGS. 2 and 3, electrons corning, from the emitter within the area bounded by the dotted line are directed by the electrodes to a defined focal point. However, in the case of the small emitter in FIG. 2, while the entire emitter is contained within the dotted line thereby achieving a narrow focal spot, a significant area that could be used to emit electrons from the emitter is not utilized, with a significant reduction in emission current. Further, in the case of a larger emitter shown in FIG. 3 that provides a greater emission current, the edges of the large emitter are outside of the effects of the electrodes, resulting in a varied focal spot such that only large focal spots can be obtained.
Accordingly, it is desirable to provide an emitter-cup x-ray tube cathode which overcomes the hereinabove described disadvantages. The importance of improved emission capabilities combined with the ability to focus higher beam currents into smaller and variably sized focal spots is clearly driven by the need to improve the image quality of the medical imaging system using current thermionic emission technology.